My composer.json:
--------------------

{
    "require": {
        "phpunit/phpunit": "3.7.*",
        "jacobsteringa/odoo-client": "0.2.1",
        "zendframework/zend-xmlrpc": "2.2 as 2.3"
    }
}

Composer.json from odoo-client:
--------------------------------

{
    "name": "jacobsteringa/odoo-client",
    "type": "library",
    "description": "A PHP Client for Odoo",
    "keywords": [],
    "homepage": "https://github.com/jacobsteringa/OdooClient",
    "license": "MIT",
    "authors": [
        {
            "name": "Jacob Steringa",
            "email": "jacobsteringa@gmail.com"
        }
    ],
    "require": {
        "zendframework/zend-xmlrpc": "2.3.*",
        "zendframework/zendxml": "1.0.*"
    },
    "autoload": {
        "psr-0": { "Jsg\\": "" }
    },
    "target-dir": "Jsg/Odoo",
    "minimum-stability": "dev"
}

Composer update output:
------------------------

Loading composer repositories with package information
Updating dependencies (including require-dev)
Your requirements could not be resolved to an installable set of packages.

  Problem 1
    - jacobsteringa/odoo-client 0.2.1 requires zendframework/zend-xmlrpc 2.3.* -> satisfiable by zendframework/zend-xmlrpc[2.3].
    - jacobsteringa/odoo-client 0.2.1 requires zendframework/zend-xmlrpc 2.3.* -> satisfiable by zendframework/zend-xmlrpc[2.3].
    - zendframework/zend-xmlrpc 2.3 requires zendframework/zend-stdlib == 2.3.0.0 -> satisfiable by zendframework/zend-stdlib[2.3.0].
    - Can only install one of: zendframework/zend-stdlib[2.3.0, 2.2.0].
    - zendframework/zend-xmlrpc 2.2.0 requires zendframework/zend-stdlib 2.2.0 -> satisfiable by zendframework/zend-stdlib[2.2.0].
    - Installation request for zendframework/zend-xmlrpc 2.2 as 2.3 -> satisfiable by zendframework/zend-xmlrpc[2.2.0].
    - Installation request for jacobsteringa/odoo-client 0.2.1 -> satisfiable by jacobsteringa/odoo-client[0.2.1].